Non Mopar carburetor opinion

So; Just sos I understand you right; It idles best with the idle mixture screws fully shutting off all fuel through their circuits.
That indicates, as Im sure you know, that the carb is supplying the idle fuel from somewhere else.Now Im pretty sure that carb has no idle fuel circuits on the secondary side. So we only have to search the primary side.
I can think of only 4 possibilities; 1)transfers, 2)mains, 3) fuel level, and 4) faulty parts
1) the transfers could be doing all the work due to the butterflies being too far open
2) the mains could be dribbling.Sometimes due to plugged anti-drool orifices, faulty accelerator pump shut-off needle, a too far open butterfly, or a damaged powervalve.
3) If the running fuel level gets too high, pull-over becomes too easy at the low speed discharge ports. Conversely; if too low, pull-over gets to be too difficult and a guy gets to cranking open the butterflies in an effort supply the craved fuel, from the transfers.
4) Faulty parts; well 2 little words to cover a multitude of possibilities! All the common areas; like warped blocks/bodies that the gaskets just cant seal,plugged/restricted air bleeds,ruptured powervalve diaphragm, stuff stuck in passages from the factory,porous or faulty passages etc..
Theres only supposed to be 4 ways for fuel to get into the intake; idle ports, transfers,mains, and pump. Barring faulty parts, One of three of those has got to be contributing when it ought not to be.(obviously not the idleports)

My money is on the transfers. And the reason would likely be a too-far open butterfly. It may be that the engine wants more air.It could also be(however unlikely) that the butterfly is too far closed, blocking the idle ports near to completely.The engine could only run this way if its getting a combustible mixture from somewhere else, and Im pretty sure it would suffer from a pretty good sized tip-in hesitation.
What I do When I get a tough case like this, and after Ive proved that the carb is fine, is this; I sync up the transfer port slot under the butterfly to appear about square to a little rectangular, with the idle speed screw. I reset the mixture screws 1.5 turns out. I close the secondaries. Then, with the engine now running, sorta,( I might have to increase the idle rpm with the fast idle cam, or a wedge between the curb idle screw and its stop) I crank in the timing to where it will stay running, and put the butterflies back on the curb idle screw. Then I begin to introduce air under the butterflies to see what the engine wants. Most want air. Unfortunately Holley et al, never included an idle air bypass circuit, so I have to figure out some way to do that. Ive had good sucess with determining the orifice size with the leak method and introducing that amount of air through the pcv system, or cracking the secondaries. Some engines didnt really like this and complained by stinking and making my eyes water and hurt. On those guys, I drilled the butterflies to get the air to pick up some transfer fuel on its way in.That seemed to do it. The important thing is to not stray too far from the initial T-port sync up.Once it gets close you can fine tune same as usual;idle speed and mixtures
Now Im not suggesting that you might have to go through all this rigmarole.Only that it sometimes comes to this.I mostly run into this on small block engines with bigger cams.I dont think your 400 will fall into this zone. I include it more for the newbes which I think will be watching this post.
I think you will find the your issue back in the 2nd paragraph.

There is a 5th possibility. I dont mention it because I know that you know all about late ignition timings.
I suppose I would be remiss,too, if I didnt at least mention the secondary side. I just assumed it was fully closed,short of sticking closed,so as not to introduce nozzle dribble on that side.
I sure hope you can find a solution in here somewhere. Cheers